In fluid flow systems it is desirable to provide a fixed orifice tube of precise form to provide a low cost device for the control of fluid flow. While such devices are suitable for their intended purpose in some cases more than one level of flow is desired which desirably should be regulated by like low cost but precise fixed restriction control.
An example of a system requiring such variable flow restriction is in the field of automotive air conditioning. Such systems include an engine driven compressor which draws refrigerant vapor from an evaporator unit over which ambient air is directed into the passenger compartment by a fan unit. The evaporator extracts heat from the ambient air by evaporating liquid refrigerant to form refrigerant vapor which is returned to the compressor. The compressor compresses the refrigerant vapor and directs it into a condenser unit where high pressure refrigerant vapor is cooled and collected as high pressure refrigerant liquid. It is common practice in such system to provide a fixed restriction such as a fixed orifice tube or capillary tube defining a high pressure liquid refrigerant line between the outlet of the condenser and the inlet of the evaporator. The conventional orifice tube in air conditioning system can work in conjunction with an accumulator/dehydrator unit to regulate refrigerant flow in the system based upon cooling demand.
In such refrigerant systems, a cycling clutch control senses the operating condition of the system at the orifice tube to turn a magnetically operated clutch on and off so as to cycle the operation of the compressor in accordance with the heat load on the refrigerant system. One disadvantage of the fixed orifice tube type systems is that compressor capacity under idle speed conditions (even when controlled to be on all the time) may be so limited because of compressor size as to be unable to provide enough flow of high pressure refrigerant liquid (or to maintain sufficient pressure differential for proper expansion) across the fixed restriction defined by the single orifice tube. Such limited capacity can cause the evaporator to run in a less efficient non-flooded condition at engine idle or when the car speed is decreased as in the case of heavy, slow moving traffic. This condition, coupled with the associated decreased condensing ability due to reduced airflow through the condenser at idle speeds, can result in a higher temperature and pressure in the liquid refrigerant. On the other hand. in some cases where there is more than enough compressor capacity, such idle operation can even further increase the heat load on the condenser which in turn can result in even higher temperatures and pressures in the liquid refrigerant. Thus, with either limited or unlimited compressor capacity at idle, such increases in pressure will tend to force more liquid refrigerant across the fixed orifice tube from the high pressure side thereof to the low pressure side of the system. With limited compressor capacity systems, such increased refrigerant flow can cause an excessive flow of refrigerant which is greater than can be pumped by the compressor. With unlimited compressor capacity systems, such increased refrigerant flow is insufficient to moderate the otherwise excessively high condenser outlet pressures.
It has therefore been suggested that multiple stage or variable flow restrictors be operated by differential pressure across the liquid refrigerant line to control the refrigerant flow in automotive air conditioning systems. Examples of such variable flow devices are set-forth in U.S. Pat. Nos. 3,296,816 and 4,375,228 assigned to the assignee of this invention. Other variable flow devices are disclosed in U.S. Pat. Nos. 920,716; 2,816,572; 3,482,415; 3,973,410; and 4,009,592.
None of the known variable orifice systems is readily adaptable to use simple mechanical valving components to produce either a decrease in high pressure liquid refrigerant flow or an increase in high pressure liquid refrigerant flow depending upon whether an associated system either has limited compressor capacity or an unlimited compressor capacity under engine idle conditions.